Organic materials have recently shown promise as the active layer in organic based thin film transistors and organic field effect transistors [see H. E. Katz, Z. Bao and S. L. Gilat, Acc. Chem. Res., 2001, 34, 5, 359]. Such devices have potential applications in smart cards, security tags and the switching element in flat panel displays. Organic materials are envisaged to have substantial cost advantages over their silicon analogues if they can be deposited from solution, as this enables a fast, large-area fabrication route.
The performance of the device is principally based upon the charge carrier mobility of the semi-conducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2 V−1 s−1). In addition, it is important that the semi-conducting material is relatively stable to oxidation i.e. it has a high ionization potential, as oxidation leads to reduced device performance.
One of the best semi-conducting polymers currently available is regioregular poly(alkyl)thiophene (PAT), with a mobility around 0.1 cm2V−1s−1. For example, regular poly(3-hexylthiophene) has been reported with a high charge carrier mobility between 1×10−5 and 4.5×10−2 cm2 V−1 s−1, but with a rather low current on/off ratio between 10 and 103 [see Z. Bao et al., Appl. Phys. Lett. 1997, 78, 2184]. In general, poly(3-alkylthiophenes) show improved solubility and are able to be solution processed to fabricate large area films. However, poly(3-alkylthiophenes) have relatively low ionization potentials and are susceptible to doping in air [see H. Sirringhaus et al., Adv. Solid State Phys. 1999, 39, 101].
There are a number of features in PAT that contribute to its high charge carrier mobility. Firstly, the polymer chains of regioregular PAT are able to pack via interdigitation of their side-chains, as schematically depicted in FIG. 1. The side-chains are necessary to provide polymers that are soluble. This results in the formation of lamellar sheets of polymers, such a arrangement is beneficial to the charge-hopping mechanism with which charge is carried in organic materials [see P. J. B. H. Sirringhaus, R. H. Friend, M. M. Nielsen, K. Bechgaard, B. M. W. Langeveld-Voss, A. J. H. Spiering, R. A. J. Janssen, E. W. Meijer, P. Herwig & D. M. de Leeuw, NATURE, 1999, 401, 685]. Secondly, PAT contains an abundance of sulfur atoms in the thiophene rings. The presence of sulfur atoms has been shown to be beneficial to charge transport, the exact mechanism is not known, but it is speculated that interaction of the sulfur d-orbitals on adjacent polymer chains facilitates the charge hopping mechanism. The major drawback of PAT, however, is that the material is oxidatively unstable. This means that the material chemically degrades in the presence of oxygen, leading to low shelf stability, and secondly the material is susceptible to doping by oxygen which results in high transistor off currents and poor transistor performance. The oxidative instability of PAT is due to the presence of many electron-rich thiophene rings in the polymer backbone, which results in a high HOMO level (around −4.9 eV).
Incorporation of benzodithiophene units in a PAT-backbone results in a polymer backbone that is less electron rich than the equivalent all-thiophene polymer, since benzene is much less electron rich thiophene.
Benzo[1,2-b:4,5-b′]dithiophene, hereinafter also shortly referred to as benzodithiophene or BDT, with the following structure (1)
has been reported in the literature to have a high charge carrier mobility and to be useful as organic semiconductor. BDT monomers or dimers, oligo- or polymers formed thereof and their use as an organic semiconductor have been described for example in Kossmehl et al., Makromol. Chem. 1983, 184(3), 627-50, Katz et al., J. Mater. Chem. 1997, 7(3), 369-76, Laquindanum et al., Adv. Mater. 1997, 9(1), 36-39 and in U.S. Pat. No. 5,625,199. For example, bis(benzodithiophene) (2) is stable up to 400° C. in air [see H. E. Katz; Z. Bao; S. L. Gilat, Accounts of Chemical Research, 2001, 34, 359].

In particular the dimeric bisbenzodithiophene (2) has been shown to exhibit high charge carrier mobilities of 0.04 cm2V−1s−1. Its structure has flatter conformation than e.g. α-sexithiophene with comparable size. This enables compressed molecular packing and strong intermolecular interactions, which is favorable for compact stacking of the material and results in π-π-overlap and hence makes this compound an effective charge transport materials with high carrier mobilities. However, bisbenzodithiophene has a very high melting point over 400° C. and very low solubility in organic solvents, so that it cannot be readily solution processed and can only be vacuum deposited.
To date some example's of a poly(benzodithiophene) substituted in the 4,8 positions have been reported. Shiraishi and Yamamoto reported polymers and co-polymers based upon alkoxy substituted BDT (3) [see K. Shiraishi; T. Yamamoto, Synthetic Metals, 2002, 130, 139-147].

However, these polymers were poorly soluble and were only investigated as potential conductive materials after doping. Only alkoxy substituted polymers were described, however, these are undesirable for semiconducting materials since the electron donating alkoxy groups results in an increase of the HOMO level of the polymer, and subsequent stability problems. Additionally the routes described to these polymers are not amenable to the synthesis of the alkyl substituted polymers described here. Pomerantz et al reported a polymer whereupon benzodithiophene was polymerized through the 4,8 positions [see M. Pomerantz; J. Wang; S. Seong; K. P. Starkey; L. Nguyen; D. S. Marynick, Macromolecules, 1994, 27, 7478-7485]. However, no examples of alkyl substituted polymers or co-polymers have been described.
It was an aim of the present invention to provide new organic materials for use as semiconductors or charge transport materials, which are easy to synthesize, have high charge mobility and good processability. The materials should be easily processable to form thin and large-area films for use in semiconductor devices. In particular the materials should be oxidatively stable, but retain or even improve the desirable properties of PAT. Another aim of the invention was to provide BDT materials that are more easily processible in the manufacture of semiconductor devices, are stable and allow easy synthesis also at large scale.
It was found that the above aims can be achieved by providing poly(benzodithiophenes) according to the present invention. These polymers still possess alkyl chains perpendicular to the polymer backbone that are able to both solubilize the polymer in organic solvents and additionally are able to form closely packed interdigitated structures. Two positions are available for the alkyl chains in the BDT polymers according to this invention: substitution in 4,8-position (3) or substitution in 3,7-position (4).

Both these positions can afford closely packed structures, as schematically depicted in FIG. 2 for the homo-polymer substituted in the 4,8 positions and in FIG. 3 for the polymer substituted in the 3,7-positions. Furthermore, like PAT, the benzodithiophene polymers have sulfur atoms in the polymer backbone. An additional advantage of the poly(benzodithiophenes) according to the present invention is that the monomers from which the polymers are synthesized are symmetrical, therefore there are no problems regarding the regioregularity of the resulting polymer. This simplifies the synthesis.